Polytrimethylene ether glycols can be produced via the acid-catalyzed polycondensation of 1,3-propanediol, optionally in the presence of comonomer diols. Polytrimethylene ether glycol produced in this manner may have quality problems, in particular the color may not be desirable to the industry. The polymer quality depends on the quality of the raw material, and the polymerization process conditions and stability of the polymer are also responsible for discoloration to some extent. Various treatment methods to improve the color of polytrimethylene ether glycols are known, and include purification or pretreatment of 1,3-propanediol prior to the polycondensation reaction. Disclosed in U.S. Pat. No. 6,235,948 is a process for the removal of color-forming impurities from 1,3-propanediol by a preheating with heterogeneous acid catalysts such as perfluorinated ion exchange polymers. Post-polymerization treatment methods with adsorbents such as carbon black, are also disclosed in the art (U.S. Pat. No. 7,294,746).
Both pre- and post-treatment methods add steps to the process for producing commercially acceptable polymer. Attempts have also been made to alter reaction conditions to control product color. For example, U.S. Patent Application Publication No. 2005/272911 discloses methods of controlling color formation by carrying out the dehydration-condensation reaction in the presence of a catalyst composed of an acid and a base, which are included in the beginning of the reaction.
There exists a need for improved and convenient methods to reduce the color of polytrimethylene ether glycol. Ideal methods would allow for polymer to be produced using standard process equipment and reagents and under reaction conditions which yield acceptable reaction times and polymers of desired molecular weights.